Typically for a sub-sea production well, the original drilling platform will have been removed and the well head will have to be accessed by means of a suitable surface vessel. In order that the required operations can be carried out to the well, it is necessary that the movement of the vessel, which is floating on the surface of the sea, be compensated, to ensure positional consistency with respect to the well itself, which is fixed on the sea bed. This is conventionally provided by means of a heave compensation system on the vessel itself which is extremely cumbersome and expensive.
GB-A-2297337 is an example of a riser system which overcomes this problem and provides compensation for the heave and swell of the sea by a continuous coiled tubing riser which extends from the surface vessel to the well and which adopts a controlled `S` profile which accommodates the movements of the surface vessel without these movements affecting the lower regions of the riser at the well head end. The disadvantage of this system is that the whole riser has to be made out of special grade materials in order to be sufficiently flexible, and this is expensive, in particular for wells which are located on a sea bed more than several hundred or several thousand feet below the sea level.
Other existing systems which involve fixed or semi-fixed platforms are expensive to install and maintain.